[PATCH] Use subprocess in gen_win.py

Using of os.popen4 in build/generator/gen_win.py probably causes
"DeprecationWarning: os.popen4 is deprecated. Use the subprocess module.".

I'm attaching the patch which fixes this problem.
I don't use Windows, so I cannot test this patch, so I'm not committing it directly.

[[[
* build/generator/gen_win.py
  (WinGeneratorBase._find_swig): Use subprocess instead of os.popen4()
   for compatibility with Python 3.
]]]

-- 
Arfrever Frehtes Taifersar Arahesis